Cancer is the second most common cause of death in the United States, accounting for 20% of all deaths. Until now, medicine has tried to overwhelm the cancer cell with brute force, slicing it out with surgery, zapping it with radiation, or poisoning it with chemotherapy. All too often, however, a few cells survive the onslaught and germinate, sometimes years later, into tumors that are impervious to treatment. If tumors can be diagnosed at early stages, it will certainly increase the survival rate of the cancer patients. Therefore, efforts are currently underway in our and various other laboratories to develop efficient tumor diagnostic imaging agents.
For many years, in vivo imaging of human anatomy was dependent upon the intravenous administration of radioactive atoms (nuclear medicine) or non-radioactive iodinated contrast media (various x-ray tests and computed tomography). However, over the last decade magnetic resonance imaging (MRI) has assumed a critical role in imaging, and, unlike x-rays or computed tomography, MR uses contrast media that contain paramagnetic ions, particularly Gadolinium [Gd(III)]. Paramagnetic ions are not themselves “seen” by the MR scanner. Rather, they affect the water in body tissue so as to increase the “signal” emitted by tissue when it is placed in a magnetic field.
By and large, MR contrast media have been neither disease-specific nor organ-specific. Injected intravenously, most are rapidly excreted by the kidneys by glomercular filtration. Although several liver-specific contrast media have been created, other organs have not been successfully targeted, and no tumor-avid MR contrast agents are available to date.
Because of the importance of detection of unknown primary tumor and metastatic disease in diagnostic oncology imaging, a tumor-avid MR contrast medium would have high implications for prognosis, therapy selection, and patient outcomes. The entire issue of cure versus palliation would be impacted.
In recent years several reports focused on certain Gd-based macrocycles as potential magnetic resonance imaging (e.g. Z. D. Grossman and S. F. Rosebrough, Clinical Radioimmunoimaging, Grune & Stratton Inc., 1988) and 99mTc or 111In chelated compounds as radiopharmaceuticals (e.g. H. D. Burns, R. F. Gibson, R. F. Dannals and P. K. S. Siegel (Eds.); Nuclear imaging in Drug Discovery, Development and Approval, Birkhauser, 1993, and G. B. Saha, Fundamentals of Nuclear Pharmacy, Springer-Verlag, 1992).
Since the approval of [Gd(DTPA)(H2O)]2− in 1988, more than 30 metric tons of Gadolinium have been administered to millions of patients worldwide. Approximately 30% of MRI exams include contrast agents, and this percentage is projected to increase as new agents and applications appear. Gadolinium is also finding a place in medical research. Over 600 references to Gadolinium appear each year in the basic science literature. While other types of MRI contrast agents, namely an iron-particle-based agent and a manganese (II) chelate have been approved, Gd(III) remains the dominant material. The reasons for this include the direction of MRI development and the nature of Gd chelates. The signal intensity in MRI stems largely from the local value of the longitudinal relaxation rate of water protons, 1/T1, and the transverse rate 1/T2. Signal tends to increase with increasing 1/T1 and decrease with increasing 1/T2. Pulse sequences that emphasize changes in 1/T1 are referred to as 1/T1-weighed, and the opposite is true for T2-weighed scans. Contrast agents increase both 1/T1 and 1/T2 to varying degrees, depending on their nature as well as the applied magnetic field. Agents such as Gadolinium (III) that increases 1/T1 and 1/T2 by roughly similar amounts are best visualized using T1-weighted images, because the percentage change in 1/T1 in tissue is much greater than that in 1/T2. The longitudinal and transverse relaxivity values r1 and r2 refer to the increase in 1/T1 and 1/T2, respectively, per milliomole of agent. T1 agents usually have r2/r1 ratios of 1–2, whereas that value for T2 agents, such as iron oxide particles, is as high as 10 or more. Advances in MRI have strongly favored T1 agents and thus Gadolinium (III). Faster scans with higher resolution require more rapid radio frequency pulsing and are thus generally T1-weighed, since the MR signal in each voxel becomes saturated. T1 agents relieve this saturation by restoring a good part of the longitudinal magnetization between pulses. At the same time a good T1 agent would not significantly affect the bulk magnetic susceptibility of the tissue compartment in which it is localized, thus minimizing any inhomogeneities which can lead to image artifacts and/or decreased signal intensity.
The other important and interesting characteristic of Gadolinium (III) chelates is their stability. They remain chelated in the body and are excreted intact. For example, the off-the shelf ligands like DTPA form complexes so stable that while the agent is in vivo, there is no detectable dissociation. Owing to their large size, lanthanides tend to favor high coordination number in aqueous media. Currently, all Gd(III)-based chelates approved for use in MRI are nine-coordinate complexes in which the ligand occupies eight binding sites at the metal center and the ninth coordinate site is occupies by a solvent water molecule.
Radiopharmaceuticals are drugs containing a radionuclide and are used routinely in nuclear medicine department for the diagnosis or therapy. Radiopharmaceuticals can be divided into two primary classes: Those whose biodistribution is determined exclusively by their chemical and physical properties (like iodine-131) and those whose ultimate distribution is determined by their biological interactions (like a radiolabeled antibody). The latter class includes more target-specific radiopharmaceuticals. A target-specific radiopharmaceutical consists of four parts: a targeting molecule, a linker, a chelating ligand and a radionuclide. The targeting molecule serves as the vehicle, which carries the radionucleide to the target site in diseased tissue. The radionuclide is the radiation source.
Metallic radionuclides offer many opportunities for designing new radiopharmaceuticals by modifying the coordination environment around the metal with a variety of chelators. Most of the radiopharmaceuticals used in conventional nuclear medicine are 99mTc labeled, because of its short half-life (6 hours) and ideal gamma emission (140 KeV). Millicurie quantities can be delivered without excessive radiation to the patient. The monoenergetic 140-KeV photons are readily collimated, producing images of superior spatial resolution. Furthermore, 99mTC is readily available in a sterile, pyogen-free, and carrier-free state from 99MO-99mTC generators. Its 6 h half-life is sufficiently long to synthesize the labeled radiopharmaceuticals, assay for purity, inject the patient, and image yet short enough to minimize radiation dose. Another radionuclide successfully used is 111In. The success of the pharmaceutical IN-DTPA-Octreotide (OCTREOSCAN), used for diagnosis of somatostatin receptor-positive tumors, has intensified the search for new target-specific radiopharmaceuticals. Compared to 99mTc, the half-life of 111In is much longer (72 hours).
Certain porphyrins and related tetrapyrrolic compounds tend to localize in malignant tumors and other hyperproliferative tissue, such as hyperproliferative blood vessels, at much higher concentrations than in normal tissues, so they are useful as a tool for the treatment of various type of cancers and other hyperproliferative tissue by photodynamic therapy (PDT) (T. J. Dougherty, C. J. Gomer, B. W. Henderson, G. Jori, D. Kessel, M. Kprbelik, J. Moan, Q. Peng, J. Natl. Cancer Inst., 1998, 90, 889). However, most of the porphyrin-based photosensitizers including PHOTOFRIN® (approved worldwide for the treatment of tumors) clear slowly from normal tissue, so patients must avoid exposure to sunlight for a significant time after treatment. In recent years, a number of chlorophyll analogs have been synthesized and evaluated for their use as photosensitizers for PDT (e.g. R. K. Pandey, D. Herman, Chemistry & Industry, 1998, 739). Among these photosensitizers, the hexyl ether derivative of pyropheophorbide-a 9 (HPPH) (e.g. R. K. Pandey, A. B. Sumlin, S. Constantine, M. Aoudia, W. R. Potter, D. A. Bellnier, B. W. Henderson, M. A. Rodgers, K. M. Smith and T. J. Dougherty, Photochem. Photobiol., 1996, 64, 194; B. W. Henderson, D. A. Bellnier, W. R. Graco, A. Sharma, R. K. Pandey, L. A. Vaughan, W. R. Weishaupt and T. J. Dougherty, Cancer Res., 1997, 57, 4000; and R. K. Pandey, T. J. Dougherty, U.S. Pat. No. 5,198,460; 1993, U.S. Pat. No. 5,314,905, 1994 and U.S. Pat. No. 5,459,159, 1995) and the hexyl-ether derivative of purpurin-18-N-hexylimide 10 (e.g. R. K. Pandey, W. R. Potter and T. J. Dougherty, U.S. Pat. No. 5,952,366, 1999) have shown high tumor uptake and minimal skin phototoxicity compared with PHOTOFRIN®. HPPH is currently in phase I/II clinical trials for treatment of various types of cancer by photodynamic therapy at the Roswell Park Cancer Institute, Buffalo, N.Y. and the results are promising.